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The symbiosis between coral and their single-celled dinoflagellate symbionts, Symbiodinium, is the foundation of reef ecosystems, and a critical element of reef resilience (van oppen and gates 2006). Corals host a diverse community of Symbiodinium, ranging along a continuum from ‘selfish opportunistic symbionts’ (e.g. some clade D Symbiodinium) which are better suited to sustained environmental stress, to ‘intimately evolved symbionts’ which provide exceptional amounts of nutrition to their coral host (17 , 18). Diversity with the genus Symbiodinium is high, described as “on the order of orders” (CITE), and is divided hierarchically into clades, subclades, and types. Symbiotic flexibility and stability can be variable both within and among coral species (Putnam et al 2012, CITE?), and this can be a driver in determining “winers and losers” (Loya et al 2001) during coral bleaching events. Coral bleaching is the breakdown of symbiosis, where Symbiodinium are expelled en masse from the tissues of their coral host. A coral’s susceptibility and resilience to bleaching is, in part, determined by fine-scale variability in their compliment of associated symbionts (Sampayo et al 2008). Corals have been observed to recover from bleaching only if the underlying stress, such as ocean warming, abates.
Ocean warming events can cause massive losses of coral cover (e.g. 17% of global coral reefs during the 1997/98 El Niño) (CITE,CITE). The 2015-2016 El Niño, superimposed on nearly-ubiquitous tropical ocean warming, instigated the third global coral bleaching event (15). Our study location, Kiritimati Atoll (Christmas Island, Kiribati, Central Equatorial Pacific, Coordinates: 2, -157.4), was at the epicenter of this extreme El Niño event. Thermal anomalies were severe on Kiritimati, rapidly exceeding NOAA Coral Reef Watch’s Coral Bleaching Alert Level 1 (4 Degree Heating Weeks, DHW, a metric of cumulative thermal stress) and Alert Level 2 (8 DHW) thresholds, reaching an unprecedented (Hoegh-Guldberg 2011) 25.7 DHW over a year-long bleaching event, demolishing most of the reef (???). Despite the massive mortality resulting from this extreme heat stress event, some corals survived.
Here, we assess coral symbiosis and survival during the massive 2015/2016 El Niño event. We tagged, sampled, and photographed the same coral colonies before, during, and immediately after the El Niño event. We assessed bleaching condition and survival for each coral colony, and used Illumina MiSeq ITS2 amplicon sequencing and 97% de novo OTU clustering to evaluate changes in Symbiodinium community structure. To investigate mechanisms underlying the ability of these corals to not only survive a year of continuous heat stress, but to recover in the interim, we assessed the relationship between human disturbance, pre-bleaching Symbiodinium community structure, and coral survival, as well as the timing of Symbiodinium community shifts throughout this El Niño event. We document, for the first time, corals that were able to visually recover from bleaching, and to regain their Symbiodinium communities during an extreme heat stress event. These corals (family Faviidae; Platygyra sp. and Favites sp.) were bleached within two months of the onset of warming, but had visibly recovered after 10 consecutive months of intense warming. Previously, corals have been shown to recover from bleaching only after the external stress (e.g. warming) has subsided (Gates and van Oppen 2016?7?), implying that longer and more frequent stressors spell disaster for reefs worldwide. This unprecedented resilience mechanism…